Sex Determination in Sciarid Flies: a Model for the Control of Differential X-chromosome Elimination

In sciarids, all zygotes start development with the 3X;2A chromosome constitution, two of the three X chromosomes being of paternal origin. The elimination of either one or two paternal X chromosomes produces the X:A signal which determines development along the female (2X;2A) or male (X0;2A) pathway, respectively. A model is proposed in which a chromosomal factor (CF) positively interacts with the X chromosome(s) causing its/their elimination. The number of X chromosomes to be eliminated is controlled by a maternal factor (MF) which regulates the amount of free CF factor interacting with the X chromosomes. Imprinting refers to the inability of maternal X chromosomes to bind CF factor.